Oxidation of low-density lipoprotein plays a key role in processes leading to the development of atherosclerosis. LDL oxidation is accompanied by alterations in its biological properties resulting in, for example, accelerated uptake through scavenger receptors in macrophages, altered chemotactic behavior of monocytes, and monocyte-derived macrophages, endothelial cell damage, and increased amounts of mediators of cell proliferation and platelet aggregation (refs. 1-4). All these effects may contribute to the development of atherosclerotic lesions. Therefore, determination of the LDL oxidation related parameters, namely LDL oxidation products, and antioxidant potential, gives more specific information on atherosclerosis-related biochemical phenomena than the commonly used measurements, of which the most common are the measurement of serum cholesterol, LDL and other lipoproteins and the apolipoproteins.
Most of The data on LDL oxidation come from studies where oxidation of LDL fractions, isolated by conventional ultracentrifugation methods, has been monitored by the appearance of conjugated dienes or thiobarbituric acid reactants arising during oxidation of isolated LDL in vitro (5). Thus far, when LDL oxidation has been investigated in humans in vivo, analyses of LDL oxidation products have been based on antibodies raised against in vitro oxidatively damaged LDL (5). The existing methodology is complex and time-consuming and, in addition, the specificity of the immunological analyses can be questioned (3). Therefore, there is still need for single rapid and specific measurement of LDL oxidation that could become part of the laboratory repertoire in the diagnosis and management of atherosclerosis (5).
The immunological methods developed for direct measurement of oxidized LDL may not be specific, as, in addition to oxidized LDL, antibodies seem to recognize also other epitopes (6) and have given contradictory results as well (3). The poor applicability of immunological methods may be a reflection of the chemistry of LDL oxidation: LDL oxidation can be initiated in various different polyunsaturated fatty acids, and each of these can give rise to a number of different kinds of oxidation products. Due to the multiplicity of oxidation products, development and use of immunological methods is likely to remain problematic also in the future.
The existing methods for measuring the antioxidant potential of LDL are complex and time consuming, and for example only a limited number of analyses can be performed within one working week: LDL is first isolated by ultracentrifugation, whereafter the samples still have to be dialyzed. Another disadvantage is the unprecise recording of results, where changes of the various reaction phases are not always easily detected.
We have developed, for the analysis of LDL oxidation parameters, namely LDL oxidation products and LDL antioxidant potential, methods which are rapid and simple to perform, and can therefore be used for large-scale clinical studies. The validity and clinical applicability of these analytical procedures is clearly indicated by several studies.